Method of fabricating multi-bit flash memory

ABSTRACT

A method of fabricating a multi-bit flash memory, having a control gate, a floating gate, a source region, a drain region and a channel region. An isolation region is formed in the floating gate to partition the floating gate into a plurality of conductive blocks. The conductive blocks are arranged in an array with rows extending from the source region to the drain region. Each row of the array has two conductive blocks. Before any data is written to the flash memory, the channel regions under the conductive blocks of the same row have the same threshold voltage, while the channel regions under the conductive blocks of different rows have different threshold voltage.

BACKGROUND OF INVENTION

1. Field of the Invention

The invention relates in general to a non-volatile memory, and more particularly, to a multi-bit flash memory and method of fabricating the same.

2. Related Art of the Invention

Among various non-volatile memories, the flash memory has become a memory device broadly applied in personal computer and electronic equipment due to the advantages of multiple data accesses and data retention after power interrupt.

The typical flash memory includes floating gate and control gate made of doped polysilicon. The floating gate is formed between the control gate and the substrate and under a floating state without electric connection to any other devices. The control gate is coupled to the word line. The flash memory further includes a tunneling oxide and a dielectric layer located between the substrate and the floating gate, and between the floating gate and the control gate, respectively. While programming the flash memory, a positive voltage is applied to the control gate, and a relatively small voltage is applied to the drain region (or source region). Thereby, hot electrons generated between the substrate and the drain region (or source region) are injected through the tunneling oxide and trapped in the floating gate. The hot electrons are uniformly distributed all over the polysilicon floating gate. Therefore, one flash memory cell can store either “1” or “0” and is a single-bit memory cell.

The increase of semiconductor integration has driven the demand for developing multi-bit memory cells. For example, in the U.S. Pat. No. 6,420,237, a method of fabricating a flash memory with multi-bit memory cells is disclosed. In this disclosure, a floating gate is partitioned into two independent blocks to form the two-bit structure. Again, the multi-bit memory cell cannot meet the high-density data storage requirement, and the memory cell able to store multiple bits is required.

SUMMARY OF INVENTION

The present invention provides a multi-bit flash memory and a method of fabricating the same. Multiple bits of data can be stored in a single memory cell of the flash memory, such that the device integration is increased.

The present invention further provides a multi-bit flash memory and a method of fabricating the same to avoid over-erase and enhance reliability of the flash memory.

The flash memory provided by the present invention comprises a control gate on a substrate, a floating gate between the control gate and the substrate, a source region and a drain region in the substrate at two sides of the floating gate, a channel region in the substrate under the floating gate, and an isolation region in the floating gate. The isolation region partitions the floating gate into a plurality of conductive blocks arranged as an array. The rows of the array extend from the source region to the drain region. Each row of the array comprises two conductive blocks, and each column of the array comprises n (n is a positive integer) conductive blocks. Before data is written into the multi-bit flash memory, the channel regions under the conductive blocks of the same row have the same threshold voltage, while the channel regions under the conductive blocks of different rows have different threshold voltages.

The above multi-bit flash memory further comprises a gate dielectric layer formed between the control gate and the floating gate, and a tunneling oxide layer formed between the floating gate and the substrate.

In the above structure, the isolation region partitions the floating gate into a plurality of conductive blocks to form the multi-bit structure, and the channel regions under the conductive blocks of different rows have different threshold voltages. Therefore, multiple bits of data can be saved in a single memory cell to increase the device data storage quantity and integration. Further, as the floating gates are partitioned into individual and independent conductive blocks (that is, the bits are separated from each other), the problem of secondary electron injection is resolved, and the device reliability is enhanced.

The method of fabricating a multi-bit flash memory provided by the present invention comprises the following steps. A silicon oxide layer and a conductive layer are sequentially formed on a substrate. An isolation layer is formed in the conductive layer to partition the conductive layer into a plurality of conductive blocks. An array formed of the conductive blocks includes rows extending from one bit line to another bit line, and columns each having n (n is a positive integer) conductive blocks. A gate dielectric layer is formed on the conductive layer. The gate dielectric layer and the conductive layer are patterned to form a floating gate. Bit lines are formed in the substrate at two sides of the floating gate. A control gate is then formed on the gate dielectric layer. A threshold voltage adjustment step is then performed to result in different threshold voltages for the channel regions under the conductive blocks of different rows.

In the above method, the conductive layer includes germanium polycide. The method of forming the isolation region includes forming a patterned photoresist layer exposing the region predetermined for forming the conductive blocks on the conductive layer, following by an ion implantation step for implanting oxygen ions (or nitrogen ion) into the exposed region. An annealing step is then performed, such that the oxygen ions (or nitrogen ion) are reacted with the silicon of the conductive layer.

The above method further comprises a step of forming a field oxide layer on the bit lines and a spacer on a sidewall of the floating gate.

Further, in the above method, the isolation region partitions the conductive layer into a plurality of conductive blocks. The channel regions under the conductive blocks of different rows have different threshold voltages, such that a single memory cell has the multi-bit structure. Therefore, without increasing the volume of the memory cell, the stored bits of data and device integration are increased. Further, as the conductive blocks are separate from each other (that is, each bit of the memory cell are independent from each other), the problem of generating secondary electron injection is resolved.

BRIEF DESCRIPTION OF DRAWINGS

These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein:

FIG. 1A shows a top view of a multi-bit flash memory in one embodiment of the present invention;

FIG. 1B shows a cross-sectional view along A—A″ of FIG. 1A;

FIG. 2 shows a top view of a multi-bit flash memory in another embodiment of the present invention;

FIGS. 3A to 3F show top views of a flash memory fabricated in various processing stages; and

FIGS. 4A to 4F are cross-sectional views along the line B—B″ as shown in FIGS. 3A to 3F.

DETAILED DESCRIPTION

The structure of the multi-bit flash memory is described with reference to the accompanied drawings. FIG. 1A (top view) and FIG. 1B (cross-sectional view) illustrate the structure of a multi-bit flash memory in one embodiment of the present invention. FIG. 2 shows the top view of a multi-bit flash memory in another embodiment of the present invention. In FIG. 2, FIGS. 1A and 1B, the same devices are denoted by the same reference numerals.

Referring to FIGS. 1A and 1B, the flash memory comprises a substrate 100, a gate structure 102, a source region 104, a drain region 106 and a channel region 107. The gate structure 102 is located on the substrate 100. The source region 104 and drain region 106 are formed in the substrate 100 at two sides of the gate structure 102. The channel region 107 is located in the substrate 100 under the gate structure 102 between the source region 104 and the drain region 106.

The gate structure 102 includes a tunneling oxide layer 108, a floating gate 110, a gate dielectric layer 112 and a control gate 114. The control gate 114 is located on the substrate 100. The floating gate 110 is formed between the control gate 114 and the substrate 100. The gate dielectric layer 112 is formed between the control gate 114 and the floating gate 110. The material of the gate dielectric layer 112 includes silicon oxide, silicon oxide/silicon nitride, or silicon oxide/silicon nitride/silicon oxide (ONO), for example. The tunneling oxide layer 108 is formed between the floating gate 110 and the substrate 100. An isolation region 116 is formed in the floating gate 110 to partition the floating gate 110 into a plurality of conductive blocks for forming the multi-bit structure. The conductive blocks are arranged as an array with rows extending from the source region 104 to the drain region 106. In the array of conductive blocks, each row comprises two conductive blocks and each column comprises n (n is a positive integer) conductive blocks. Before data is written into the multi-bit flash memory, the channel regions under the conductive blocks of the same row have the same threshold voltage, while the channel regions under the conductive blocks of different rows have different threshold voltages. In this embodiment, a 2×2 array with four conductive blocks (110 a, 110 b, 110 c and 110 d) are exemplarily described. Therefore, under the status without any data written in the flash memory, the channel regions under the conductive blocks 110 a and 110 b of the first row have the same threshold voltage. Similarly, the channel regions under the conductive blocks 110 c and 110 d of the second row have the same threshold voltage. However, the channel regions under the conductive blocks 110 a and 110 b have a threshold voltage different from that of the channel regions under the conductive blocks 110 c and 110 d.

In the above structure, the isolation region 116 partitions four conductive blocks 110 a, 110 b, 110 c and 110 d, while the channel regions under the conductive blocks 110 a and 110 b have different threshold voltage from the channel regions under the conductive blocks 110 c and 110 d. Therefore, four bits of data can be saved in a single memory cell to enhance device integration. Further, these four conductive blocks (that is, these four bits) are independent from each other, such that the secondary electron injection is avoided, and device reliability is enhanced.

In the above structure, the floating gate 110 is partitioned into four conductive blocks 110 a to 110 d as an example. It is appreciated that the floating gate 110 can also be partitioned into more than four conductive blocks (such as six conductive blocks 110 a to 110 f as shown in FIG. 2). By adjusting the threshold voltages of the channel regions under the conductive blocks of different rows to be different from each other, a six-bit memory structure is formed.

The structure of a multi-bit flash memory is described as above. Hereinafter, the method of fabricating the multi-bit flash memory is introduced. FIGS. 3A to 3F are top views of the flash memory formed in various process stages, and FIGS. 4A to 4F are cross-sectional views along the line B—B″ as shown in FIGS. 3A to 3F.

In FIGS. 3A and 4A, a substrate 200 is provided. The substrate 200 includes a silicon substrate, for example. An oxide layer 202 is formed on the substrate 200 as a tunneling oxide layer. The method of forming the oxide layer 202 includes thermal oxidation, for example.

A conductive layer 204 is formed on the oxide layer 202. The material of the conductive layer 204 includes germanium polycide, for example. The method of forming the conductive layer 204 includes chemical vapor deposition with silane, germane and hydrogen as reaction gases.

Referring to FIGS. 3B and 4B, a patterned photoresist layer 206 is formed on the conductive layer 204. The patterned photoresist layer 206 exposes a region of the conductive layer 204 predetermined for forming an isolation region.

An ion implantation step 208 is performed using the patterned photoresist layer 206 as a mask. The dopant implanted into the exposed region include oxygen ions. Oxygen ions are implanted into the exposed region to form the oxygen ion doped region 210. The dosage of the oxygen ions is about 1×10¹⁸ atoms/cm² to about 2×10¹⁸ atoms/cm², and the implantation energy is about 20 KeV to about 80 KeV, for example. In an alternative embodiment, the dopant implanted into the exposed region also includes nitrogen ions or other ions, which can react with silicon to form insulation material.

Referring to FIGS. 3C and 4C, the patterned photoresist layer 206 is removed, and an annealing process is performed. The oxygen ions (or nitrogen ion) and the silicon contained in the conductive layer 204 are reacted into silicon oxide (or silicon nitride) to form the isolation region 212. The annealing process is performed at about 950° C. to about 1150° C. The isolation region 212 partitions the conductive layer 204 into a plurality of conductive blocks, for example, the conductive layer 204 of a single memory cell is partitioned into four conductive blocks in this embodiment.

A gate dielectric layer 214 is formed over the substrate 200. The material of the gate dielectric layer 214 includes silicon oxide, silicon oxide/silicon nitride, silicon oxide/silicon nitride/silicon oxide, for example. The thickness of the gate dielectric layer 214 is about 50 angstroms to about 150 angstroms, for example. The method for forming the gate dielectric layer 214 includes chemical vapor deposition, for example.

Referring to FIGS. 3D and 4D, a patterned photoresist layer 216 is formed on the gate dielectric layer 214. The patterned photoresist layer 216 exposes a part of the gate dielectric layer 214 under which bit lines are formed subsequently. The exposed part of the gate dielectric layer 214 and the underlying conductive layer 204 are removed to expose the substrate 200 predetermined for forming the bit lines. An ion implantation step 218 is performed using the patterned photoresist layer 216 as a mask to form doped regions 220 (bit lines) in the substrate 200 exposed at two sides of the patterned photoresist layer 216. The implanted ions include arsenic ions with a dosage of about 2×10¹⁵ atoms/cm² to about 4×10¹⁵ atoms/cm² and an implantation energy of about 50 KeV.

Referring to FIGS. 3E and 4E, the patterned photoresist layer 216 is removed. A thermal process is performed to form a field oxide layer 222 on the doped regions (bit lines) 222 and activate the dopant therein. The field oxide layer 222 is formed to isolate the doped regions 222 and the control gate (word line) formed subsequently. While forming the field oxide layer 222, a spacer 224 may be formed on a sidewall of the conductive layer 204. The spacer 224 provides isolation between the conductive layer 204 and the control gate (word line) formed subsequently.

A conductive layer 226 is formed on the substrate 200. The material of the conductive layer 226 includes doped polysilicon, for example. The method for forming the doped polysilicon includes in-situ implantation and a chemical vapor deposition process, for example.

Referring to FIGS. 3F and 4F, a mask (not shown) is used to pattern the conductive layer 226, so as to define the control gate 228 (the word line). While patterning the conductive layer 226, the same mask is used to define the gate dielectric layer 214, the conductive layer 204 and the oxide layer 202 for forming a gate structure, which uses the patterned conductive layer 204 as the floating gate 230. Consequently, the gate structure comprises a stacked structure of the control gate 228, the gate dielectric layer 214, the floating gate 230 and the oxide layer 202. The floating gate 230 of each memory cell comprises four separate conductive blocks 204 a, 204 b, 204 c and 204 d arranged in a 2×2 array.

A step of adjusting threshold voltage is then performed. A patterned photoresist layer 232 is formed over the substrate 200 to expose the control gate 228 over the conductive blocks 204 c and 204 d. Ion implantation is performed using the photoresist layer 232 as a mask to implant dopant into the channel regions 234 b under the conductive blocks 204 c and 204 d, so as to adjust the threshold voltage of the channel regions 234 b. Therefore, the threshold voltage of the channel regions 234 b under the conductive blocks 204 c and 204 d is different from that of the channel region 234 a under the conductive blocks 204 a and 204 b. A single memory can thus store four bits of data at once. The subsequent process is known to the person of ordinary skill in the art, and is not further described.

In the above embodiment, oxygen ions are implanted into the conductive layer 204 to partition the conductive layer 204 into a plurality of conductive blocks, so as to form a multi-bit structure. Therefore, without increasing the volume of the memory cells, more bits of data are stored in each memory cell, and the device integration is increased. Further, the conductive blocks are independent and separate from each other, so that the problem of secondary electron injection is resolved.

The dopant implanted into the exposed region not only include oxygen ions, but also include nitrogen ions or other ions which can react with silicon to form insulation material.

In addition, the isolation region 212 may also be formed to partition the conductive layer 204 into another number of conductive blocks instead of four conductive blocks. By adjusting the channel regions under the conductive blocks of different rows with different threshold voltages, various multi-bit structures are obtained.

Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. A multi-bit flash memory, comprising: a substrate; a control gate on the substrate; a floating gate between the substrate and the control gate; a gate dielectric layer between the control gate and the floating gate; a tunneling oxide layer between the floating gate and the substrate; a source region and a drain region in the substrate at two sides of the floating gate; an isolation region in the floating gate to partition the floating gate into a plurality of conductive blocks arranged in an array with a plurality of rows extending from the source region to the drain region and a plurality of columns, wherein each row comprises two conductive blocks and each column comprises n (n is a positive integer) conductive blocks; and a channel region in the substrate under the floating gate between the source and drain regions, wherein before writing data into the multi-bit flash memory, the channel region under the conductive blocks of the same row has the same threshold voltage, and the channel region under the conductive blocks of a different row has different threshold voltages.
 2. The multi-bit flash memory according to claim 1, wherein the material of the gate dielectric layer includes silicon oxide.
 3. The multi-bit flash memory according to claim 1, wherein the material of the gate dielectric layer includes silicon oxide/silicon nitride.
 4. The multi-bit flash memory according to claim 1, wherein the material of the gate dielectric layer includes silicon oxide/silicon nitride/silicon oxide.
 5. The multi-bit flash memory according to claim 1, wherein the material of the floating gate includes germanium polycide.
 6. The multi-bit flash memory according to claim 1, wherein the isolation region includes a silicon oxide layer.
 7. The multi-bit flash memory according to claim 1, wherein the isolation region includes a silicon nitride layer. 